1. Field of the Invention
The present invention relates to waterborne one-component binder compositions for the preparation of sandable, stone chip resistant and weather resistant coatings, preferably for automotive OEM finishing, which dry at temperatures below 100xc2x0 C., and to the preparation and use thereof.
2. Description of the Prior Art
In automotive OEM finishing, a multi-layer composition of the top coats has become generally established and is also being obtained to an increasing extent with the use of waterborne coating compositions. These coatings also have to meet economic requirements. The objective is to apply the coatings in as few and as inexpensive individual steps as possible.
In the conventional coating compositions for automotive OEM finishing, a stone chip resistant layer and a primer surfacer layer or a combination of the two (stone chip resistant primer surfacer) is applied initially to a metal surface primed by cathodic electro-dipcoating. A base coat followed by a clear coat or alternatively a pigmented top coat is then applied to these layers.
The stone chip resistant and/or primer surfacer layer serves to smooth out surface irregularities and brings about good stone chip resistance due to high elasticity and deformability. Flexible polyester or polyether polyurethanes and polyisocyanate or melamine crosslinking agents have previously been used for this layer. Before the application of base coat and clear coat or top coat, the stone chip resistant primer surfacer is stoved. This is necessary for improving the condition of the top coat and for partially sanding the primer surfacer layer before application of the upper coating layers. After the clear coat or top coat has been applied, stoving is carried out once again. A disadvantage of this process is that two expensive stoving processes at temperatures of  greater than 120xc2x0 C. are required. Coatings drying at lower temperatures, of the kind that may be formulated, for example, from polyacrylates, do not exhibit the required stone chip resistance since they crosslink to form brittle films.
There is a need for storage-stable, one-component coating compositions which, apart from good stone chip resistance, have good sandability and surface hardness. The coating should also be lightfast in order to produce a weather resistant coating even in the case of moderately hiding top or base coats or in areas where a pigmented top or base coat is dispensed with entirely. The coating should also adhere to different substrates, such as the plastics frequently used in automobile construction (pretreated polypropylene or polyethylene, pretreated polypropylene/EPDM blends, polyurethane-RIM, ABS, polybutylene terephthalate, polycarbonate, polyamide, SMC, BMC and others, and blends of the plastics mentioned), and also ferrous and non-ferrous metals and cured coating layers with chemically different binders. Finally, in view of the low heat resistance of the thermoplastics frequently used in automobile construction (see above), the coating composition should be curable or crosslinked at temperatures below 100xc2x0 C.
A proposed solution for stone chip resistant coatings and/or primer surfacer layers based on waterborne binders is described, e.g., in EP-A 0 330 139. It is claimed that a polyacrylate may be added to an OH- and COOH-functional polyester. No corresponding embodiments are provided, however. Graft copolymerization of the polyacrylate onto a polyester as the main chain is not described either. The claimed dispersions of acid-functional polyesters are well known to have poor storage stability since they are subject to rapid chemical degradation due to cleavage of ester bonds (e.g. Jones, T. E.; McCarthy, J. M., J. Coatings Technol. 76 (844), p. 57 (1995)).
EP-A 0 024 727, for example, describes a stoving coating composition based on a combination of epoxy resin-phosphoric acid ester, a water-thinnable polyester and water-soluble melamine resins. DE-A 4 000 748 describes, in embodiments, waterborne automotive primer surfacers based on water-thinnable hydroxy-functional polyester-polyurethane resins, optionally other binders and water-soluble amino resins.
The stringent requirements which the automotive industry in particular places on primer surfacer coatings are not yet fully met by these coatings. An improvement was obtained by more refined, polyisocyanate-crosslinked primer surfacer coatings (M. Bock, H. Casselmann, H. Blum xe2x80x9cProgress in Development of Waterborne PUR Primers for the Automotive Industryxe2x80x9d, Proc. Waterborne, Higher Solids and Powder Coatings Symp. New Orleans 1994). All the systems mentioned, however, have the disadvantage that the water-thinnable polyester or polyester-polyurethane resins used have limited storage stability if they are crosslinked with reactive amino- and/or polyisocyanate resins for lower stoving temperatures.
EP-A 0 980 880 describes stone chip resistant, physically drying coatings produced using polyacrylate dispersions in combination with polyurethane dispersions. Said coatings have insufficient sandability, however.
WO PCT/00/01205 describes chip resistant, physically drying coatings produced using polyacrylate dispersions. These combinations, however, exhibit shortcomings in the stone chip resistance at temperatures below 0xc2x0 C.
An object of the present invention was, therefore, to provide storage-stable, waterborne, one-component, lightfast coating systems having a low co-solvent content which have improved sandability and good stone chip resistance at stoving temperatures of  less than 100xc2x0 C. It is also an object of the present invention for the coating composition to provide adhesion to the various substrates used in automobile construction.
Surprisingly, it was found that these objects may be achieved by the use of a combination of special hard and highly elastic waterborne polyurethane (PU) dispersions containing virtually no free OH groups, and highly reactive water-thinnable melamine resins.
The present invention relates to a storage-stable, waterborne, one-component, coating composition, which can be cured at temperatures below 100xc2x0 C. to provide a stone chip resistant and sandable coating, containing as the binder a polyurethane dispersion A, which after physical drying results in a coating having a pendulum hardness according to DIN 53 157 of  greater than 90 s; a polyurethane dispersion B, which after physical drying results in a coating having a pendulum hardness (DIN 53 157) of  less than 90 s; and a melamine resin C, which is preferably highly reactive.
The present invention also relates to a process for coating substrates with these coating compositions and to the resulting substrates.